(i) Technical Field
The present invention relates to a fixing device, and an image forming apparatus.
(ii) Related Art
In image forming apparatuses, such as a copying machine and a printer using electrophotography, a photoconductor (photoconductor drum) formed in the shape of, for example, a drum is uniformly charged, the photoconductor drum is exposed to controlled light based on the basis of image information, and an electrostatic latent image is formed on the photoconductor drum. After the electrostatic latent image is formed as a visible image (toner image) with toner and the toner image is transferred from the photoconductor drum to a recording paper, the toner image is fixed on the recording paper by a fixing device.
A fixing device 900 used for the image forming apparatus, as shown in FIG. 8, is constituted by a fixing roll 110 that includes a heating source 113 inside a cylindrical core and that is formed such that a heat-resistant elastomer layer 111 is laminated at the core and a release layer 112 is laminated at an outer circumferential surface, and a pressure roll 120 that is arranged so as to come into pressure contact with the fixing roll 110 and that is formed such that a heat-resistant elastomer layer 122 is laminated at a core 121 and a release layer 123 that is made of heat-resistant resin coating or heat-resistant rubber coating is laminated at an outer circumferential surface. A toner image is fixed on a recording medium (for example, recording paper) by passing the recording medium holding an unfixed toner image between the fixing roll 110 and the pressure roll 120 and performing heating and pressurization on the unfixed toner image. Such a fixing device is called a roll-nip type, a two-roll type, or a heating roller type, and is generally widely used.
Incidentally, when speed-up is attempted in the heating roll type fixing device, it is necessary to increase the nip width of a nip portion formed between the pressure roll and the fixing roll in proportion to a fixing speed so that a sufficient amount of heat may be supplied to the toner and the recording medium. As methods of increasing the nip width, there are a method of increasing the load between the fixing roll and the pressure roll, a method of increasing the thickness of the elastomer layer of the fixing roll, a method of increasing respective roll diameters, and the like.
However, in the method of increasing the load and the method of increasing thickness of the elastomer layer, the shape of the nip width resulting from deflection of the rolls may become nonuniform along a roll axis. Thus, uneven fixing may occur, creases may be generated on the recording medium, and problems may occur in terms of image quality. Additionally, in the method of increasing the diameter of the rolls, there are problems in that the enlargement of the apparatus is caused, and the time (warm-up time) until the temperature of the rolls is raised from a room temperature to a fixable temperature becomes long.